Topic:
Bend-Insensitive Fiber

Optical fiber is sensitive to stress, particularly
bending. When stressed by bending, light in the
outer part of the core is no longer guided in the
core of the fiber so some is lost, coupled from the
core into the cladding, creating a higher loss in
the stressed section of the fiber. If you put a
visible laser in a fiber and stress it, you can see
the light lost by the stress as in this multimode
fiber.

Fiber coatings and cables are designed to prevent as
much bending loss as possible, but it's part of the
nature of the fiber design. Bending losses are a
function of the fiber type (SM or MM), fiber design
(core diameter and NA), transmission wavelength
(longer wavelengths are more sensitive to stress)
and cable design.

In 2007, a new type of "bend-insensitive" singlemode
fiber was introduced, followed by multimode fiber in
2009. Manufacturers liked to demonstrate this fiber
by bending it around impossibly small bends or
stapling it to a piece of wood - demonstrations that
made veterans of the business cringe at seeing fiber
treated
so badly!
But the demonstrations showed that these fibers
could be bent in what seemed like impossibly small
radii without significant light loss, although
skeptics still wondered about the long term effects
of this kind of abuse on reliability.

Once the patents were filed, manufacturers were more
willing to explain what they had done to make these
fibers so tolerant to tight bends. An optical
"trench" - the term used for a ring of lower index
of refraction material - was built into the fiber to
basically reflect the lost light back into the core
of the fiber. It turns out, the design was similar
to a type of singlemode fiber called "depressed
cladding" fiber that was first introduced in the
late 1980s.
And in 2009, manufacturers introduced multimode
fibers that showed using using a similar technique
could also improve bending loss in multimode fiber.
Let's examine the design of bend-insensitive
multimode fiber (which we will usually call by its
acronym BI MMF) that shows the technique.

In regular graded index multimode fiber, there are
many modes (or rays of light - about 400 of
them) being transmitted down the fiber. The
inner modes are "strongly guided" which means they
have little sensitivity to bending stresses. But the
outer modes are "weakly guided" which means they can
be stripped out of the core when the fiber is bent.

Bend-insensitive fiber adds a layer of glass around
the core of the fiber which has a lower index of
refraction that literally "reflects" the weakly
guided modes back into the core when stress normally
causes them to be coupled into the cladding. Some
early singlemode fibers (depressed-cladding fibers)
used a similar technology to contain the light in
the core of the fiber but this design has a much
stronger effect.

The trench, or moat as some people call it,
surrounds the core in both BI SMF and BI MMF to
reflect lost light back into the core. The trench is
just an annular ring of lower index glass
surrounding the core with very carefully designed
geometry to maximize the effect. See the red ring
around the core on this fiber drawing.

When you look at the end of a bend-insensitive fiber
in a microscope with angled lighting, you can
sometimes actually see the trench as a gray ring
around the core.

File
courtesy of Panduit

Bend-insensitive fiber
(or BI fiber as it is now called)
has obvious advantages. In patch panels, it should
not suffer from bending losses where the cables are
tightly bent around the racks. In buildings, it
allows fiber to be run inside molding around the
ceiling or floor and around doors or windows without
inducing high losses. It's also insurance against
problems caused by careless installation.

BI fibers are available in 50/125 MM (OM3 and OM4)
and SM versions. Considering the advantages of
BI fiber and the small incremental cost to
manufacture it, some
manufacturers have decided to make all their 50/125
MM fiber bend-insensitive
fiber.

Many applications for BI SM fiber are in premises
installations like apartment buildings or for
patchcords, where it simplifies installation and
use. BI SMF is also used in OSP cables since it
allows fabrication of smaller, lighter high fiber
count cables.

Compatibility With Conventional Fibers
One question that often arises is are these fibers
compatible with regular fibers. Can they be spliced
or connected to other conventional (non-BI) fibers
without problems? How does the inclusion of higher
order modes affect bandwidth? That answer seems to
be yes for all SM fibers. Since only one mode is
guided in the core, the trench has a minimal impact
on system performance and measurement. It seems you
can mix and match regular and BI SMF fibers with no
problems.

For MM fibers, it is less clear. Measurement of core
size, NA, differential mode delay (DMD) and
bandwidth were developed prior to the introduction
of BI MMF designs. These measurements are in the
process of being evaluated and updated so
measurement results may depend on the
manufacturer of the BI MMF. For the most part, it
appears that BI MM fiber can be made to be
compatible to other non-BI fibers by modifying the
core design slightly or careful engineering of the
trench surrounding the core, but at this point it is
left to the manufacturers to show their product will
perform equivalently to the installed base of fiber.

When short lengths of BI MMF are measured, they may
have a larger effective NA and core size than
conventional MM fibers since they propagate "leaky
modes" that are attenuated in conventional fiber
designs. This may affect splice or connector loss
when mating BI MMF with conventional
MMF but usually only in one direction, from BI MMF
to conventional
MMF, in a manner similar to the losses
from mismatched fibers.

One approach to make BI MMF compatible with non-BI
fibers is to modify the core index profile slightly
to reduce the higher order modes to make them
compatible to conventional
fibers without otherwise materially affecting the
performance of the fiber. A second approach is to
leave the core index profile alone but carefully
engineer the trench to produce the
bend-insensitivity.
If you want to read more on the past controversy
about compatibility of BI MM fibers, see the FOA
Newsletter for February, 2011.

Testing
Testing BI MM fibers or using them for reference
cables for testing is another matter. Most multimode
fiber testing standards call for modal conditioning,
often using a mandrel wrap mode filter (http://www.thefoa.org/tech/ref/testing/test/MPD.html).
The mandrel wrap specified in most standards doesn't
affect BI fibers the same way as conventional
fibers, so either special sources or very small
mandrels are required. The encircled flux standard
for modal fill which is being adopted by many new
standards may address some of these concerns.

Another problem arises when testing mixes of BI MM
and conventional fibers with an OTDR. Some BI fiber
has a larger scattering coefficient than regular
fibers and can create large directional loss
differences when testing splices or connectors with
an OTDR, causing gainers in MM fiber OTDR tests, a
major confusion. This issue has not been addressed
yet by all fiber manufacturers or standards, so
check with your fiber supplier.See
the FOA
Newsletter for February, 2012.

Always check with your fiber vendor for their
recommendations.

FOA thanks three major manufacturers of optical
fiber for their contributions to this page.